Chapter 2 · Acids, Bases and Salts

NCERT Class 10 Science Chapter 2 Intext Questions (Page 22)

All 3 questions of NCERT Class 10 Science Chapter 2 Intext Questions (Page 22) — Acids, Bases and Salts — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   Why should curd and ​sour substances not be ​kept ​in ​brass and copper ​vessels?
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   Acids in curd and sour foods react with the vessel's copper and zinc, forming poisonous salts that spoil the food.

   Curd and sour substances contain acids (lactic acid in curd; citric, tartaric or acetic acid in sour foods). Brass (copper and zinc) and copper are metals, and acids react with metals to form a salt and hydrogen gas: Acid + Metal $\to$ Salt + Hydrogen. The copper and zinc salts so formed dissolve into the food; they are poisonous, spoil the taste and colour of the food and make it unfit for eating. So curd and sour substances should not be kept in brass or copper vessels.

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2. Q2
   Which ​gas ​is usually liberated when an ​acid reacts with a metal? Illustrate ​with ​an example. How will you test ​for the presence ​of this gas?
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   Hydrogen gas is liberated, e.g. when zinc reacts with dilute sulphuric acid; it burns with a pop sound near a flame.

   The gas usually liberated is hydrogen (${\text{H}}_2$). Example: zinc reacts with dilute sulphuric acid to give zinc sulphate and hydrogen, $\text{Zn}(s)+{\text{H}}_2{\text{SO}}_4(aq)\to {\text{ZnSO}}_4(aq)+{\text{H}}_2(g)$ (Activity 2.3). Test: pass the gas through soap solution so that it fills soap bubbles (or collect it in a test tube) and bring a burning candle or matchstick near it — hydrogen burns with a 'pop' sound, $2{\text{H}}_2+{\text{O}}_2\to 2{\text{H}}_2\text{O}$.

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3. Q3
   Metal ​compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved ​extinguishes ​a ​burning candle. Write ​a balanced chemical ​equation for the reaction ​if one of the compounds formed is calcium chloride.
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   A is calcium carbonate; the gas is carbon dioxide, and ${\text{CaCO}}_3+2\text{HCl}\to {\text{CaCl}}_2+{\text{H}}_2\text{O}+{\text{CO}}_2$.

   The gas is carbon dioxide (it extinguishes a burning candle), so A is a metal carbonate; since calcium chloride is formed, A is calcium carbonate, ${\text{CaCO}}_3$ (limestone, chalk or marble). Balanced equation: ${\text{CaCO}}_3(s)+2\text{HCl}(aq)\to {\text{CaCl}}_2(aq)+{\text{H}}_2\text{O}(l)+{\text{CO}}_2(g)$.

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